docs(kanban): describe delegated runtime attachment

Replace the north-star walkthrough's listener snippet with prose and guard example docs against teaching addEventListener glue.

req: examples/003

req: runtime/001

slhx-test/tests/examples_contract.rs

@@ -117,6 +117,7 @@ fn canonical_example_docs_do_not_teach_low_level_plumbing() {
         "render_html(",
         "SafeHtml::trusted",
         ".render(&",
+        "addEventListener(",
     ];
     let mut failures = Vec::new();
     scan_examples(&examples, &mut |path, text| {